This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 02079284.2 filed in Europe on Oct. 9, 2002, which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a multicolor ink jet printing method, wherein a carriage is moved relative to a recording medium in a main scanning direction and a subscanning direction orthogonal to the main scanning direction, and each color is printed with a different group of nozzle heads mounted on the carriage, and wherein at least one color is printed with a larger number of nozzle heads than the other colors. The present invention further relates to a printer for carrying out the present method.
2. Related Art
A typical multicolor ink jet printer for printing, for example, with the four colors yellow (Y), magenta (M), cyan (C) and black (K), comprises at least four nozzle heads, i.e., at least one for each color. Each nozzle head has a row of nozzles arranged in the subscanning direction. Thus, when the carriage is moved back and forth across the recording medium in the main scanning direction, each nozzle head prints a swath on the recording medium during each pass of the carriage. In a single-pass printing method, the recording medium, e.g. paper, is fed in the subscanning direction by an amount corresponding to the width of the swath after each pass of the carriage. When the nozzle heads are aligned in the main scanning direction, the different color components are superimposed one upon the other during each pass, and the desired hue of the image is obtained by subtractive color composition.
However, a so-called “banding” phenomenon may occur because the order in which the color components are superimposed depends on the direction of movement of the carriage, and a change in this order leads to slight differences in the obtained hue of the printed image, and, as a result, the hue differences are visible in the form of bands or stripes on the printed image. This problem may be overcome by staggering the nozzle heads in the subscanning direction, so that the different color components are printed in different passes of the carriage and the order in which the colors are superimposed will always be the same, irrespective of the direction of movement of the carriage.
A similar banding phenomenon may also occur when two or more nozzle heads for the same color, e.g. black, are staggered in the subscanning direction in order to increase the printing speed. In this case, banding is due to minor differences in the properties of the different nozzle heads, e.g. differences in the optical densities obtained therefrom. This phenomenon can be mitigated by employing a multi-pass printing method. In a two-pass mode, for example, each printhead prints only every second pixel in each line in a first pass, and the gaps are filled-in by the second nozzle head in the subsequent pass. As a result, the pixel patterns produced by the two nozzle heads are interleaved, and the differences in the properties of the nozzle heads are smoothened out, so that a high image quality is achieved, although at the cost of reduced production.
U.S. Pat. No. 6,257,699 discloses a printing method and a printer of the type indicated above. The printer has three staggered nozzle heads for the colors yellow, magenta and cyan, and another group of three nozzle heads for black. The nozzle heads for black are mounted on the carriage in reverse orientation as compared to the other three nozzle heads and are also staggered in the subscanning direction, such that the staggered rows of nozzle heads overlap in the subscanning direction and one of the nozzle heads for black is aligned with the nozzle head for cyan in the main scanning direction. This printer can be operated in different print modes, including (a) a multi-pass color printing mode, wherein only one of the three black nozzle heads is used in combination with the nozzle heads for the other three colors, (b) a single-pass black and white printing mode, wherein all three black nozzle heads are used and the other nozzle heads are disabled, and (c) a mixed mode, in which black is printed in a multi-pass mode while the other colors are printed in a single pass mode.
WO-A-00/58102 discloses a printer having four nozzle heads for the colors yellow, magenta, cyan and black aligned in main scanning direction and one additional nozzle head for black which is offset from the other nozzle heads in the subscanning direction. The two black nozzle heads are used to speed-up black and white printing. When printing with all four colors, only the offset black nozzle head is used, so that black is printed in another pass than the other three colors. The purpose of this measure is to reduce smears by providing a larger time delay between the time at which a pixel is printed in yellow, magenta and/or cyan and the time at which a black dot is superimposed on this pixel.